The production of hollow bodies in the form of honeycombs is known. For this purpose, specifically profiled plastic pipes or profiles are connected in such a way that a cross-sectional honeycomb structure is produced which may have a diameter and length of several meters. These hollow bodies are mainly used for gas purification in which the air volume is axially passed through the honeycombs resulting in a substantial enlargement of the contact and exchange surface between the gas and the pipe wall.
Particular fields of application for hollow bodies of this type are so-called wet-type electrostatic filters in which harmful substances passing through the honeycomb pipes are deposited at the walls via a voltage field produced by center electrodes. The material used for structures of this type has to meet high requirements in view of the chemical and thermal stresses during the process and the required resistance to electric flashovers or arcings which are possible in the high-voltage field.
At present, the prior art publications describe the production of honeycomb structures of the above mentioned type on the basis of PVC, extruded hexagon pipes or edged PVC sheets being connected so as to form hexagonal honeycomb structures. The characterizing feature of these structures is the fact that, in order to save space, the walls of the honeycomb structure are adjacent to each other without leaving a space. In practice, this is effected by bonding the honeycomb pipes. As a result, the hollow body has a very stable shape which is typical for honeycomb structures and guarantees the narrow dimensional tolerances required for wet-type electrostatic filters when various thermal and mechanical stresses occur.
A honeycomb structure of this type is disclosed in DE-A1-41 02 732.
Polyvinyl chloride is a particularly suitable material for this honeycomb structure since, in addition to the chemical resistance, it guarantees a high dimensional stability and the capability of being bonded which is a prerequisite for the production.
On the other hand, the durability of wet-type electrostatic filters is restricted by the fact that voltage flashovers from the centrically arranged electrodes to the PVC walls cannot be excluded, resulting in considerable local burning of the walls.
It turned out that the plastics material polypropylene does not have this considerable drawback of the PVC. However, despite this advantage, this material has specific drawbacks as regards its processing into honeycomb structures. In particular, polypropylene has a low dimensional stability under the influence of heat due to the high memory effect in connection with the very high thermal expansion coefficient of the material.
A further disadvantage of PVC is its problematic production and disposal from the environmental point of view.
In contrast to this, the production, processing and disposal of polypropylene is uncomplicated because it is not harmful to the environment.
However, extruded profiled pipes and hot-formed or edged and welded polypropylene profiles which can be used for producing honeycomb structures exhibit high internal stresses resulting in considerable dimensional stability problems when they are processed into honeycomb structures.
A further drawback of the material polypropylene is the fact that it cannot be bonded. As a consequence, the usual prior art production methods are ruled out.
DE-B2 26 41 114 discloses a method of producing a plastic electrostatic filter in honeycomb form. The honeycombs are composed of hot-pressed plate strips in the form of sheet-piling which consist of glass-reinforced polyester. Glass-reinforced polyester was selected because PVC shows too high relaxation effects in case of temperature stress. In order to prevent the drawbacks of pure plastics, this known method uses glass-reinforced polyester.
EP-A2-0 153 681 discloses a method of producing three-dimensional transparent honeycomb structures of plastics. In this publication, plastic films or plates are formed into rib-like structures by heat-forming or deep-drawing. Subsequently, the films or plates produced are stacked upon another to form a honeycomb structure.
It is known per se from Kunststoff-Lexikon ("Manual of Plastics") (published by Carl Hanser Verlag, Munich, Vienna 1981), page 504, to produce thermoplastic plastics also by injection molding.
DE-C1-41 41 934 relates to an electric separator comprising collecting electrodes arranged in a honeycomb structure.
Said collecting electrodes form a star-shaped component consisting of three wall parts hermetically connected with each other. At the outer edges of each wall part pipe sections are located in the longitudinal direction into which rods are introduced which connect the star-shaped components with each other and thus form a honeycomb structure. The star-shaped components consist of electrically conductive plastics. In this connection, the fact that the components are connected by the pipe sections is considered advantageous because joining processes such as welding or bonding can be dispensed with.
DE-A1-27 25 220 relates to a group of dishes for cultivating and growing plants. In this publication, a basic structure in the form of honeycombs is disclosed whose center part consists of a rotting or decomposing special paper or carton. This allows the honeycomb structure to split up when plants are cultivated.